SV40 nucleoprotein complexes have been extracted from virus infected cells to follow the replication and maturation of viral DNA. Four principle forms of SV40-protein complexes have been detected: (1) A heterogeneous collection (75S-200S) of replicating DNA molecules, (2) a 75S nuclear protein complex that is the immediate product of replicated viral DNA, (3) a 200S previrion that is derived from the 75S complex and (4) a 250S virion derived from the previrion complex. Pulse-chase and kinetic labeling experiments have been used to establish the precursor product relationship between these various forms of SV40-protein complexes. Structural studies of the nucleoprotein complexes and the proteins they contain are underway.